The difference between its and it’s (from last year)

Yesterday in Fiction 2, Lila decided we needed a brief tutorial in the proper usage of some common grammar mistakes. One of those was the difference between its and it’s. She asked if I could use the possessive “its” in a sentence, and this was our response:

me (surprised, having been thinking wicked things about the boy sitting across from me): uh, fuck… it ate its own head!
Lila (without missing a beat): thank you. Correct!
(my peers giggle a little)
Stefani: Did that just come to you?